Attach a cat to the bread prior to buttering. Since cats always land on their feet, the bread is certain to land butter side up. Remember to unattach the cat before eating, however, or things could get messy.

Buttered Toast: which side does it fall on?
There were two things being tested here: 1. If buttered toast falls off the table, does it prefer to land butter side down 2. If a buttered toast falls through the air, which side does it prefer to fall on?

First (Adam's) rig: Adam's rig most closely replicated a piece of toast falling off of a table top. Testing with a control sample of unbuttered test, the dominant behavior was for the toast to flip once and land top side down. They didn't need to do any more testing with actual buttered toast, as the rig clearly had a bias.

Second (Jamie's) rig: Jamie's rig tested whether or not, all things being equal, which side toast prefers to fall on. It shoots toast straight down.

With control sample testing, toast kept landing down. Once again they were statistically challenged, as they stopped after 10 samples. They determined that 3 ups and 7 downs was enough to show a clear down bias, and once again, if just one of those had been different, they it would have been 4 ups and 6 downs, which doesn't seem biased at all.

Third rig: based on Jamie's original design, but with way more over-engineering to be more automated, regular, and MythBuster-y. A conveyor belt toaster dropped the toast off onto a second conveyor belt that carried toast over to Adam, who marked the toast and loaded it into a dropper that was then released with a switch.

11 up and 13 down with control sample

12 up and 12 down with buttered sample

They determined this to be "less biased", so they then brought it to the roof of MythBusters HQ. From the top of the roof:

26 up and 22 down with control sample

29 up and 19 down with buttered sample

Jamie's theory was that for a lot of the buttered toast that landed butter side up, the buttered side was pressed in, forming a cup that affected the way the toast dropped. Regardless: